1. Field of the Invention
The present invention relates to a technique for restricting noise in image data. More particularly, the invention relates to a technique for restricting granular noise present in photographic image data obtained by digitization of an image, in particular, such data obtained by a film scanner.
2. Description of Related Art
Generally, a standard photographic film has a grain (pixel) density on the order of 2500 dpi. The commonly employed 135 F sized photographic film has a photographic image area of: 36 mm×24 mm. Hence, its photographic image has total of 3445×2362 pixels (grains). On the other hand, a film scanner employed in the most recent digital image printing system referred to as “digital mini-lab” has an image resolution exceeding 2000 dpi. This means that the film scanner can input a photographic image from the photographic film with a resolution substantially equal to that of the film. Hence, the printing system using such scanner can make a print which reproduces a photographic image with substantially same image resolution as that of the original, i.e. the film. For quality improvement of the photographic image, it is needed to effect a so-called sharpness enhancing operation for enhancing the contour in the photographic image. However, if this sharpness enhancing operation is effected on photographic image data obtained by such high-quality film scanner with the grain (pixel) level resolution equivalent to that of the photographic negative, the operation enhances not only the photographic image contour, but also graininess of the grains of the photographic film, thus resulting in unsightly image, depending on the image characteristics of the photographic image. Such enhanced graininess leading to unsightly image is referred to herein as “granular noise.” The granular noise, especially if present in a human skin area, is a significant disadvantage to the photographic image quality. To reduce such granular noise, a blurring (smoothing) operation will be effected. This operation results, however, in blurring of the contour, in addition to granular noise reduction, thus compromising the effect of the sharpness enhancing operation effected previously.
As a solution to the above problem, there is known a technique (from e.g. Japanese Patent Application “Kokai” No. 2003-132352 (see its “Abstract” and FIG. 1)), comprising the steps of: obtaining from image data sharpened image data with enhancement of each pixel of the data; obtaining also from the image data smoothed image data with smoothing of each pixel of the data; setting a sharpness-mixing ratio correlation such that the mixing ratio of the smoothed image data will be increased for a most frequently occurring value of sharpness calculated for each pixel of the image data and obtaining corrected data for the image data by mixing the sharpened image data and the smoothed image data for each pixel in accordance with the sharpness-mixing ratio correlation. Namely, in this technique, sharpness of the image is detected and then, based on this detected sharpness, the sharpening operation and the smoothing operation are effected selectively. However, as it is difficult to determine the mixing ratio between the sharpening operation and the smoothing operation, this technique has not fully solved the problem.
As a different solution, another technique is known (from e.g. Japanese Patent Application “Kokai” No. 2002-44473 (paragraphs [0024]-[0028] and FIG. 1)). In this technique, source image data is separated between density data and color data; then a ratio between a smoothing operation for color data and a smoothing operation for density data is varied in accordance with variation of the density data in two-dimensional coordinate space. In doing so, in view of the fact that the data relating to a contour of the image is contained more in the density data than in the color image data, as the image data moves from a flat (even) area of the image toward a contour area of the same, a ratio of density noise removal is progressively decreased to ‘0’ (zero) and also the density noise removing operation is terminated earlier than the color noise removing operation. In essence, this technique effects the smoothing operation selectively on a flat area of the image. If a conventional sharpness enhancing operation is effected thereafter as a post operation, it is possible to obtain enhanced sharpness at the image contour with certain restriction of granular noise. However, the contour line has a certain width due to inaccurate focus or a shade, the above technique will result in weak smoothing on the contour line per se and/or its periphery. Hence, the granular noise will remain at such portion. Further, as the smoothing operation uses a spatial filter of n×n pixels, even if only one pixel included in the calculating area of the spatial filter constitutes the noise, such smoothing operation as above will result in an image in which the effect of the noise pixel is “extended” over to the entire area of the spatial filter. As a result, this technique tends to result in unnecessarily flat (even) image.